The present invention relates to drilling apparatus used in the exploration of underground petroleum reserves and more specifically to a rotary drill bit having a plurality of rotary cutters supported for rotation on the drill bit body by a plurality of rolling elements, the rolling elements being laterally guided at least at their axial ends facing the drill bit body in a radially disposed outer bearing race groove incorporated in the bore of the rotary cutter.
Rotary drill bits having conically shaped rotary cutters are known wherein the rotary cutters are supported on a trunnion formed integrally with the drill bit body and a radial roller bearing. The rollers of this bearing are laterally guided in a bearing race groove in the bore of the rotary cutter and supported in place at their axial ends facing away from the drill bit body by means of a flange bolt secured in the trunnion so that each rotary cutter is mounted in a manner preventing removal from the trunnion. An arrangement of this type is shown in the British Pat. No. 456,570. This known assembly has several disadvantages and drawbacks. For example, the construction requires that the flange bolt be shaped in the form of a race element for the axial bearing of the rotary cutter and be made of an expensive bearing material. In order to mount the flange bolt in a precise manner in the trunnion, the bolt has to be machined with close tolerances. Thus, the method of manufacturing is relatively costly. Additionally, a center bore with a relatively large diameter must be provided in the trunnion to receive the highly stressed flange bolt and accordingly, the trunnion of the drill bit body is somewhat weakened and consequently there is the risk of trunnion failure by breakage especially in heavy duty operations.
In accordance with another known rotary drill bit assembly, the rotary cutters are each supported on a trunnion connected in one piece with the drill bit body in a radial roller bearing with conical roller elements. This arrangement is shown in U.S. Pat. No. 2,076,000. This arrangement has the advantage of providing a trunnion which is relatively rigid and possesses good load bearing characteristics. However, a so-called edge ring is required which is located in the bore of the rotary cutter on the side of the roller elements facing the drill bit body which axially guides the roller elements to support the rotary cutter in place on the trunnion. It has been found that the fabrication of the rotary cutter is rather expensive because of the incorporation of the additional edge ring. Furthermore, in heavy duty operations with high impact stresses, it has been observed that a loosening of the edge ring may result and accordingly, presents the disadvantage that the rotary cutter is not adequately secured against being drawn off the trunnion.